TolQ

Structure
TolQ is a polytopic protein located in the inner (cytoplasmic) membrane, containing approximately 230 amino acids. There are three membrane spanning segments in TolQ with each segment containing an α-helix. TM1 is connected to TM2 via a large cytoplasmic loop, with the N-terminal ending in the periplasm. This topology is also found in ExbB, which can be explained by the similar sequence shared by both proteins.

It has been shown that 4 to 6 TolQ molecules associate in the TolQRA complex, and that they form multimers which interact with the transmembrane helices (TMH) of TolQ, TolR and TolA. The multimers are formed by the three TMHs of TolQ, the last of which undergo a conformational change to form a hairpin, while the first TMH forms an intermolecular interaction.

Function
TolQ plays an important role in the maintenance of the bacterial envelope integrity as well as the import of filamentous bacteriophage and group A colicins.

Loss of TolQ function can be partially replaced by ExbB and vice versa - reduced activities of either of these proteins via a mutant form can be reversed by introducing double mutants.